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Old August 18th 05, 01:22 PM
 
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On 17 Aug 2005 19:13:03 -0700, wrote:

I don't know what you mean by "small resistor" in series with the V+ line.
Each amplifier stage should have a series resistor of 100 ohms and a bypass
capacitor of .01uF.


I've tried several values, 220,330,510, ect. with 0.05uF bypass.
Doesn't make much difference.

As far as physical distance for each stage, you should allow 1/2 inch for
every 40dB of gain. You shouldn't have this type of problem. A ground plane
is always a good idea,


The IF cans are spaced 1 inch apart and I disconnected the 2nd IF stage
so I only have an oscillator, mixer and single IF amp stage. Same
problem, it always oscillates when the voltage is raised to obtain
reasonable gain.

Although it is possible if you have quite a bit of copper foil tape around the
board, you can build this circuit on perfboard but you have to be very careful,
as you have already discovered.


I took a look inside a AM/FM clock radio I have and checked the
ground connections. There are 3 RF transformers, oscillator, mixer and
single IF stage, and all of the metal shields of the transformers are
isolated. There are no physical connections between the shields of the
3 transformers.

Obviously, I am missing something. Grounding all the transformer
housings on a massive ground plane does not seem to be the answer.

-Bill


I will assume good wiring, lead dress and stage bypassing have been
attended to as even small amounts of feedback regardless the cause
will insitgate oscillation.

Transistors have internal capacitance from collector to base and in
high impedence RF amplifiers that serves to couple input to output
and make a fine oscillator. The gain attainable before oscillation
was usually listed as MUG (max usable gain) and is usually specified
with a test circuit. Often that circuit was neutralized (more later)
to allow best gain without oscillation. For a .455khz IF a stage gain
of more than around 35DB from one transistor is begging for
instability.

There are several solutions for that. One is reduce the gain, not
always a good solution but sometimes required. Another is to
neutalize the amplifier. Neutralization is to feed back some of the
signal out of phase back to the base lead to balance out the signal
comming back via the collector base capacitance. Like anything
enough is good too much is bad.

As it turns out most ransistor radios of the 50s up till the do it all
on a chip era had to cope with unintentional feedback. Most applied
both of the mentioned solutions, reduce the per stage gains and apply
neutralization.

As to how to apply neutalization... Varies depending on circuit.
Assuming your using common IF cans and generic transistor circuit
it's usually easy. The output IF can has three terminals on the
tuned side, one is meant for DC power, another is collector and the
remaining is the out of phase feedback to the base via a capacitor
in the range of 1 to 10pf. Which pin is which, depends of the can and
who made it. Another way if the IF can is not tapped is to feed
back some fo the secondary winding to the base. The tricky part here
is if the secondary is backward it will make oscillation worse and you
will have to reverse the seconday IF transformer leads. What you are
shooting for is a stable amplifier.

One of the side effects is that if the applied neutralization is too
small the stage will have some regeneration but not oscillate
and that is also unstable though a very high gain state to be in.
Strive for stability. The stage should not oscillate at any
reasonable voltage or tuning adjustment.

All of the proceeding is predicated on the transistor stage
oscillating at the IF frequency. It is possible due to various
factors that the transistor is oscillation at VHF and a number of
other frequencies as well. This is trickier, as you need to
suppress that and then deal with the IF oscillation if still present.
VHF oscillation can be killed with a 100ohm resistor in series with
the collector lead. The resistor and the collector lead should be
short. There is no measurable gain lost at IF frequencies to that
resistor as it's size is very small compared to the load.


Having built more than few radios with high gain IF stages I've had
to deal with this problem more than once. Hope it helps.

Allison
KB1GMX